The correlation between global climate change and the carbon sink by weathering of continental rocks has remained an international frontier of research. It is widely accepted that only silicate weathering can form long-lasting carbon sink and might control climate change over the long geological time scale. On the other hand, carbonate weathering doesn’t have such an effect because all the CO2 consumed during the weathering process returns back to the atmosphere through relatively rapid precipitation of carbonates in oceans. However, the above mentioned traditional view adopted for several decades, is now challenged by the latest results from a Sino-foreign cooperative research.
For over twenty years, Prof. LIU Zaihua from the Institute of Geochemistry, Chinese Academy of Sciences (IGCAS) and Prof. Wolfgang Dreybrodt from Germany (a renowned karst dynamics expert) and others conducted a collaborative study involving a great deal of field data analysis, laboratory simulation and the theoretical analysis. They have discovered that the rapid kinetics of carbonate dissolution, the significant effect of small amount of carbonate minerals in silicate watersheds in controlling the dissolved inorganic carbon (DIC), coupled with the aquatic photosynthetic DIC uptake lead to an atmospheric CO2 sink from carbonate weathering of 0.477 Pg C/a which was severely underestimated previously by a factor of about 3. This suggests 94% of the atmospheric CO2 sink by rock weathering was contributed by carbonate weathering, while silicate weathering only contributed 6%. Therefore, the study indicates, besides controlling the short-term climate change concerned by human, the carbon sink by carbonate weathering might also play a major role in controlling climate change over the long geological time scale since the occurrence of aquatic photosynthetic organisms. This undoubtedly questions the traditional view that only the carbon sink by silicate weathering controls long geological time scale climate change.
The new finding on the carbonate weathering atmospheric CO2 sink is important as indicated below.
First, the origin of carbonates in oceans and lakes since the occurrence of aquatic photosynthetic organisms must be re-investigated. Namely, is it mainly due to the silicate weathering or the carbonate weathering?
Because of the much faster kinetics of carbonate weathering than that of silicate weathering, the carbonates in oceans and lakes since the occurrence of aquatic photosynthetic organisms may originate mainly from the carbonate reaction.
Secondly, the atmospheric CO2 sink by rock weathering since the occurrence of aquatic photosynthetic organisms must be reassessed. Based on the above research, it appears that the atmospheric CO2 sink by rock chemical weathering should mainly originate from the carbonate dissolution and subsequent aquatic photosynthetic uptake and burial of the resulting DIC.
Lastly, natural aquatic ecosystems produce organic carbons through photosynthesis, which goes into the lithosphere by sedimentation and burial. This implies that the atmospheric CO2 sink by carbonate weathering is significant in controlling not only our most concerned short-term climate change but also the long-term climate change.
The study has been published in the latest issue of "Applied Geochemistry" and "Quaternary Sciences", and the reviewers commented "this paper has made significant contribution to the debates on concentrationchanges of the atmospheric CO2."
(Provided by ZENG Haitao, Translated by XIAO Yi, Edited by LIU Zaihua)